The present invention relates to the processing of refuse, and more particularly to a method and apparatus for recovering fuel and other resources from solid municipal and industrial refuse utilizing disk screens.
In the past solid municipal and industrial refuse has been disposed of by incineration and by using the refuse as landfill. In recent years the problem of refuse disposal has become critical as a result of a rapid increase in population combined with a significant increase in per capita production of waste. Landfill operations have become increasingly undesirable due to the dwindling supply of suitable acreage within a reasonable distance of population centers. This tends to make incineration the preferred alternative. In view of the current energy crisis efforts have been made to utilize refuse as a source of fuel for power plant boilers, as contrasted with merely incinerating the combustible refuse for purposes of physical reduction. In addition, efforts have been made to recover other valuable resources such as glass, aluminum, and ferrous metals so that they can be recycled. An example of one process for recovering fuel and other resources from municipal and industrial refuse is disclosed in U.S. Pat. No. 4,113,185 issued Sept. 12, 1978.
Refuse processing systems heretofore known have typically included a plurality of components for separating the refuse into individual fractions consisting primarily of combustible organic material, aluminum, ferrous metals, glass, and miscellaneous bulky inorganic material. Efficient resource recovery depends upon separating the maximum amount of desirable material from the refuse using relatively few separating components. It also depends upon minimizing the percentage of unwanted materials in the individual fractions. For example, it is desirable to produce a fraction consisting primarily of aluminum and containing very little glass, paper, plastic, dirt, etc. so that the aluminum can be readily recycled. Also the presence of incombustibles such as inorganic materials and the like in the fuel fraction can reduce the BTU content. It will also increase the ash content and necessitate the frequent cleaning of the traveling grate or suspension burning mechanisms of power plant boilers.
Conventional separating components which have been utilized in refuse processing systems in the past include screens, vibrating tables, air classifiers, cyclones, pulpers, and magnetic separators. It has been found that the combination of one or more screens with an air classifier can greatly improve the separating efficiencies of most refuse processing systems. Two basic kinds of screens have been utilized in refuse processing systems in the past. The first kind comprises a vibrating grate having apertures through which suitably sized pieces of refuse pass. The second kind is generally referred to as a trommel screen. It comprises an elongate cylinder having a plurality of apertures through its wall. Refuse is introduced into the interior of the cylinder through one of its open ends and suitably sized pieces of refuse pass through the apertures as the cylinder is rotated.
However both of the aforementioned kinds of screens have a tendency to become partially blinded fairly rapidly when used to separate shredded refuse. Their apertures become partially obstructed with refuse thus inhibiting proper grading or sifting. This in turn reduces the efficiency of the other downstream separating components. For example it has been discovered that a failure to remove a large percentage of ground glass and other fine inorganic materials will reduce the efficiency of a downstream air classifier in separating shredded light organic material from denser inorganic material. Also, the operating efficiency of downstream magnetic separators is reduced if a large percentage of paper and other organic material is not removed ahead of time. Even worse is the fact that both of the aforementioned kinds of screens eventually become totally blinded, i.e. their apertures become completely plugged with refuse. The operation of the processing system must be periodically interrupted so that these screens can be cleaned.
Disk screens having a plurality of interleaved rotating disks have been used to separate particulate material such as pulp chips from wood chunks, frozen lumps, etc. with a high degree of efficiency. They do not have a tendency to become blinded. U.S. Pat. No. 631,093 teaches that the spacing between the disks can be varied according to the quality of material to be separated. U.S. Pat. No. 4,037,723 suggests that disk screens can be used in refuse processing. However, to date a method and apparatus for processing refuse utilizing disk screens has not been developed.